Nowadays, a semiconductor memory is used in various fields such as a main storage of a large computer, a personal computer (PC), home electric appliances, and a cell phone. The rapidly growing market is a Flash-E2PROM type nonvolatile memory represented by a NAND flash memory, and various memory cards (a secure digital (SD) card, a multimedia card (MMC), a memory stick (MS) card, and a compact flash (CF) card) on which the NAND flash memory is mounted are used for a digital camera, a digital video, a music player such as a mpeg-1 audio layer 3 (MP-3) player, a storage medium of a mobile PC and the like, and a storage medium of a digital television set and the like as a medium that stores therein information on an image, a moving image, a sound, and a game. Moreover, a universal serial bus (USB) enabled card is also widely used as a storage medium for a PC.
The Flash-E2PROM type nonvolatile memory mainly includes a NOR type and a NAND type. The NOR type can perform high-speed reading; however, the effective bandwidth in writing is small, so that it is not suitable for file recording. On the other hand, the NAND type can be highly integrated compared with the NOR type, and moreover can perform a burst read operation and the effective bandwidth in writing is high although the access speed is a little slow. Therefore, the NAND type is used in the memory cards described above and a USB memory, and is recently used in a memory of a cell phone and the like.
As described above, the NAND-type nonvolatile memory can realize large capacity and high integration and therefore is often considered to replace a hard disk. However, the NAND-type flash memory is a semiconductor memory that needs erasing processing before writing, and the life thereof depends on the number of times of rewriting. Moreover, because an erasing unit is large, in the typical use environment for a PC in which data to be handled is not always large-capacity data such as image data and a music file, fatigue of the NAND memory is accelerated.
Therefore, in the case of configuring a secondary storage with large capacity by using the NAND memory, a cache memory such as a random access memory (RAM) is often placed between a flash memory and a host device to reduce the number of times of writing (the number of times of erasing) in the flash memory as described in Japanese translation of PCT international application No. 2007-528079.
The secondary storage using such a NAND memory often includes a driving circuit that drives the NAND memory and a RAM and a microprocessor unit (MPU) that controls the driving circuit (for example, Japanese Patent No. 3688835). In the conventional technology, because the NAND memory and the RAM are controlled by one MPU and one driving circuit, a problem arises in performance as a high-end storage for a server or the like.
Embodiments provides a memory system capable of applying to a high-end storage for a server or the like.